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Can anyone point me to anything that proves (or disproves) the "fact"
that unleaded mogas deteriorates faster than avgas?
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I believe this may have started with some poster way back looking at the
shelf life recommendations that come from the oil companies. Of course,
they will say gasoline lasts 3 weeks and you should drain your tank,
throw it out, and buy new stuff if you keep it longer. Their whole angle
is to avoid liability. Similarly, someone (mechanics, FAA, Mogas STC
producers, whoever) made some statements about the "limited" shelf life
of mogas vs. avgas. Opponents to mogas seem happy to spout off about the
"dangers" of mogas, including their conclusion that it "goes bad quickly".

Those with chemical backgrounds look over the ingredients lists for
avgas and mogas and come to the conclusion that a shorter shelf life for
mogas makes sense.

The question Jay asks is: where is the scientific data that shows how
long these fuels last when stored before bad stuff happens? I'll bet the
answer is "it depends". Probably depends on the time stored,
temperatures, amount of humidity in the air, water present in the fuel,
how much agitation, specific blend of fuel and additives, sunspots,
solar flares, karma, aliens... who knows?

Likely also that fuels change slowly over time. So, what is "bad"? Not
sure we have an established baseline for when this mysterious threshold
is reached. Airplane folks will likely agree that fuel that won't start
their engine qualifies. How about fuel that starts the engine and kills
it later, at altitude, gums up the fuel system, clogs the screens over
time, vapor locks when hot, etc.? Is that "bad"?

It is likely there are enough variables to skew the storage numbers
quite a bit. And, all the stories about how one person does this and it
works fine and another person does that and it turns to crud means...
well, nothing.

As Jay requests, bring on the data.

Avgas does smell better...

Have fun beating this to death,
Mike